This invention relates to power transistor drive circuits.
With the advent of wide-angle, large-screen television kinescopes, the amount of deflection power handled by the deflection circuits has increased considerably.
As a result, the horizontal output transistor is highly stressed and is therefore subject to premature failure. In order to reduce the thermal stresses on the horizontal output transistor, it is normally operated in a switched mode rather than in a linear mode. Switched operation reduces the power dissipated in the transistor and the temperature rise resulting from the dissipation. Generally speaking, the faster the horizontal output transistor can be switched, the less power is dissipated in the transistor during each switching cycle. Attempts to increase the switching speed of the transistor encounter the storage of charge carriers in the base region as a limitation. The transistor continues to conduct until reverse bias is applied to the base-emitter junction with a magnitude and for a period of time sufficient to sweep the base region free of stored charge carriers.
In order to provide an initial peak in the reverse bias current to minimize the time required to sweep the charge carriers from the base region of the output transistor, it is customary to use a transformer to couple a driver transistor to the output transistor. One consideration is that the peak value of the reverse-biasing drive signal portion cannot be maintained as bias after the output transistor turns OFF, because the power dissipated in the base-emitter region would be significantly and undesirably increased, and thereby offset the desired reduction of power dissipated in the output transistor. Therefore, the reverse bias after initial turn-off of the output transistor must be below the peak value.
An improved arrangement for decreasing the turn-off time of the horizontal output transistor is described in U.S. Pat. No. 3,302,033 issued on Jan. 31, 1967 in the name of H. C. Goodrich. This arrangement uses pulse-forming networks in conjunction with a transformer to provide the output stage with a reverse-bias pulse of suitable magnitude during the initial portion of the turn-off interval as well as with suitable magnitude during the remainder of the reverse bias interval.
The use of multiple pulse-forming networks is described in an article "Design Factors for Transistorized Television Deflection Circuits" by C. Frank Wheatley, Jr., published in the IEEE Transactions on Broadcast and Television Receivers, July, 1963, pages 72 to 80. In the arrangement of this article, two resistance-capacitance pulse-forming networks charged from a single voltage source are coupled together by a diode, and the drive transformer is energized by the coupled networks when the driver transistor goes ON. This circuit provides even more drive circuit design flexibility for providing the large value of the peak output transistor reverse base current pulse, together with a relatively small reverse current thereafter, for reduced output transistor dissipation and reduced drive power.
The drive transformer is an alternating-current coupling mechanism, and therefore the total charge flow in the secondary winding during the reverse-bias condition of the output transistor must equal the total charge flow during the forward-bias interval, which is to say during the second half of the trace interval. It has been discovered that it is not possible to specify the optimum peak value of the reverse bias current independently of the magnitude of the optimum remaining reverse-bias and forward-bias currents with the prior art arrangement because the resistance-capacitance values should provide the proper time constants for drive pulse waveshaping as well.